First evidence of mesospheric hydroxyl response to electron precipitation from the radiation belts

Utilizing observations from the Medium Energy Proton and Electron Detector (MEPED) on board the Polar Orbiting Environmental Satellite (POES) and the Microwave Limb Sounder (MLS) on board the Aura satellite, we demonstrate that there is a strong link between 100-300 keV loss cone electron count rates observed in the outer radiation belt and nighttime OH concentrations in the middle mesosphere at 71-78 km altitude. In theory, this can be expected because the ionization caused by energetic electron precipitation (EEP) leads to odd hydrogen (HOx) production through ionic reactions. However, this is the first time that OH production due to EEP has been observed. We consider daily mean data from 2 months, March 2005 and April 2006, which were selected because of (1) relatively high count rates of radiation belt electrons observed and (2) the absence of solar proton events that could mask the EEP effects. The results show that at 55-65 degrees magnetic latitude (equivalent to McIlwain L shells 3.0-5.6) increases in electron count rates by 2 orders of magnitude are accompanied by increases in nighttime OH concentration of 100%. There is a high correlation between MEPED and MLS data such that 56-87% of the OH variation can be explained by changes in EEP. Because the relation between MEPED count rate observations and the flux of electrons actually entering the atmosphere is not trivial, we discuss the possibility of using OH observations to obtain an estimate of EEP forcing that could be used in atmospheric modeling.